The networking of control devices, sensors, and actuators with the help of a communication system, i.e., a bus system, has drastically increased in recent years not only in the construction of modern vehicles but also in mechanical engineering, in particular in the machine tool sector and in automation. In the process, synergy effects can be achieved by distributing functions to a plurality of control units. These are the distributed systems. The communication between different users of such distributed systems is increasingly taking place over a bus or a bus system. The communication traffic on the bus system, access and receiving mechanisms, as well as error processing, is regulated via a protocol.
The CAN (controller area network) is the established protocol in the automotive sector. This is an event-controlled protocol, i.e., protocol activities, such as sending a message, are initiated by events having their origin outside the communication system. Unique access to the communication system or bus system is achieved through priority-based bit arbitration. This is on condition that each message is allocated a unique priority corresponding to the ID. The ID in the CAN is uniquely allocated here to the message content of a data transmission. The CAN protocol is very flexible, making the addition of other nodes and messages possible without any problem as long as there are still free priorities, also called IDs or message identifiers. The collection of all messages with priorities and their transmitting nodes, and possibly receiving nodes, to be sent in the network, are stored in a list, the communication matrix. CAN systems are often designed such that there is a unique allocation of the IDs to particular preselectable users.
The bit-synchronous interface, BSS, is known as another protocol on a bus system. It is used, for example, as a communication protocol between control unit and generator regulator for set-point regulation and status display, and also as a diagnostic interface for vehicle workshops, etc. The structure of the BSS protocol corresponds to that of a master/slave protocol. A master communicates with up to a maximum of 8 slaves that are connected via a linear bus line. An unshielded physical single core line serves as a transmission medium for the pulse-width modulated bits. The master transmits synchronizing impulses at all times, which can be overwritten by information bits, and, as the sole bus user, is able to initiate a data exchange. Through one message, the master can communicate with only one slave at any given time. In the BSS protocol, for example, a message telegram includes 3 synchronizing bits and 19 information bits. Before each message, at least 3 synchronizing bits are sent, for instance, in order to facilitate a synchronizing of the slave. These synchronizing bits, as initiator of the data transmission, are consequently transmitted by the master as part of the data transmission on the bus system.
As with BSS, LIN (local interconnect network) is a master/slave bus whose users are connected via a logical bus line. A maximum of one master and up to 64 slaves belong to each bus. An unshielded physical single core line serves as transmission medium. The LIN protocol divides the sending and receiving process into two tasks. The master task contains the synchronization and addressing, and the slave task contains the data. Both master task and slave task are summarized in a MessageFrame. A master can execute the master task as well as the slave task. Each data transmission has a MessageFrame, in which the synchronization as well as the data information are transmitted. Byte fields, the individual sections of the message, have the same format as a serial interface in an 8N1 transmission. This means 8 data bits, no parity bit, and 1 stop bit. At the start of every message, the master sends a SynchBreak as a synchronization signal in order to fetch the slaves from a possible idle state and to facilitate synchronization. Here, the SynchBreak includes two different parts: a first part, a bus-dominant signal of fixed duration, in particular 13 bits long. The second part corresponds to a bus-recessive signal of fixed duration, in particular 1-bit.
As in the examples mentioned, there is a plurality of bus systems that can be used as a communication link between users, in particular in distributed systems. Protection from unauthorized access to these systems is gaining more and more importance here. In bus systems in motor vehicles today, the data is for the most part transmitted without protection. If the bus line is connected by an unauthorized user, it can send commands that trigger the release of the central locking mechanism, for example, and consequently, an unlocking of the vehicle. In general, such unauthorized access to a bus system is associated with a high hazard potential, for example, in automation or in anti-theft protection in motor vehicles.
Nevertheless, in order to ensure a certain amount of protection from unauthorized access, the bus line is installed only in places where it is especially difficult for an intruder to tap the bus line. In especially critical applications, e.g., starting the vehicle engine, the data is also sent encrypted, as a rule. The encryption of the data is often associated with a very high cost. Nevertheless, a majority of the algorithms used may be decoded by an unauthorized user with a certain amount of effort and knowledge.